The present invention relates to a method of non-contact thermometry and, more particularly, to a method of thermometry based on measuring the variation in time of induced infrared emissions of the target.
According to Planck's law, a material body at a temperature T radiates electromagnetic energy according to the following formula: EQU Q(.lambda.,T,.epsilon.)=.epsilon.C.sub.1 .lambda..sup.-4 /(exp(C.sub.2 .lambda..sup.-1 T.sup.-1)-1) photons sec.sup.-1 cm.sup.-1 .mu.m.sup.-1
where .lambda. is the wavelength of the electromagnetic radiation, .epsilon. is the emissivity of the body, C.sub.1 and C.sub.2 are constants, and Q is the number of photons emitted into a hemisphere per unit wavelength per unit area per unit time. The emissivity .epsilon. depends on the wavelength .lambda., the temperature T, the geometry of the body, and the nature of the surface of the body.
Passive measurements of the radiation emitted by a target long have been used to measure the temperature of the target. These measurements have the advantage over other methods, such as the use of thermocouples, that the measuring device need not be in direct physical contact with the target. These methods suffer from several disadvantages, however, notably that the emissivity .epsilon. of the target generally is unknown. Examples of such passive radiometric techniques intended to cope with this problem by making measurements at several wavelengths include those of Brower et al., in U.S. Pat. No. 4,659,234, and Tank, in U.S. Pat. No. 4,924,478.
Active methods commonly are used to measure physical properties of the target. These active methods can be implemented in two ways. The first way is implemented by periodically modulating laser radiation. The second way, known as "pulsed photothermal radiation" (PPTR), is implemented by using a pulsed laser. In both methods, the radiation from the laser is absorbed by the target, causing a local and temporary increase of the surface temperature of the target. The resulting increase in emitted radiation, and the subsequent decrease in emitted radiation as the target cools, are measured and recorded, using a suitable detector. This record constitutes a function of time, the "photothermal decay curve", which is analyzed to infer information about the target's physical properties. These active methods have the following advantages over passive radiometry:
1. The spatial resolution of active radiometry is determined by the size of the portion of the target that is heated. If a laser is used to heat the target, this size is substantially the size of the laser spot on the target. The size of the laser spot can be on the order of the wavelength of the laser radiation. Therefore, spatial resolution can be very high.
2. In the case of weakly radiating targets, the signal-to-noise ratio can be increased by increasing the heat applied to the target.
3. Because only the time-varying part of the signal is measured, these methods are immune to both thermal and electronic drifts.
4. For the same reason, these methods are not biased by reflected fluxes that are constant in time. Therefore, they may be applied to targets in hot surroundings and to targets with high surface reflectance.
5. The temporal resolution of these methods, particularly PPTR, can be increased by decreasing the pulse duration. Because measurements can be made in short times, measurements can be made on moving targets, such as turbine blades, and on transient targets, such as superconductors undergoing phase transitions.
For example, Egee et al., in U.S. Pat. No. 4,875,175, disclose an active radiometric method for measuring physical parameters of a layered material. Egee et al. use periodically modulated laser radiation to heat a portion of the target. A similar method (Thierry Loarer, Jean-Jacques Greffet, and Magdeleine Huetz-Aubert, "Noncontact surface temperature measurement by means of a modulated photothermal effect", Applied Optics, Vol. 29 No. 7, Mar. 1, 1990, which is incorporated by reference for all purposes as if fully set forth herein) has been used for thermometry. Like periodically modulated active radiometry, PPTR has been applied to thermometry (Thierry Loarer and Jean-Jacques Greffet, "Application of the pulsed photothermal effect to fast surface temperature measurements", Applied Optics, Vol. 31 No. 25, Sep. 1, 1992, which is incorporated by reference for our purposes as if fully set forth herein).
Loarer, Greffet, and Huetz-Aubert infer the temperature of their target from the absolute magnitude (amplitude) of the dynamic portion of the photothermal signal. Loarer and Greffet infer the temperature of their target from the absolute magnitude of the total integrated photothermal signal measured during a sampling time window. Their methods therefore suffers from the disability that they must be calibrated separately for each acquisition geometry and target emissivity. They use the dual bandpass radiometry technique, a technique well-known in the literature, in which the temperature is inferred from the ratio of photothermal signals measured at two different wavelengths. However, this technique suffers from several drawbacks. For example, the complexity of the measurement system is increased by the need for two detectors, and the measurement is independent of target emissivity and acquisition geometry only when there are no changes, or identical changes, in the optical transmittance of the two separate channels.
There is thus a widely recognized need for, and it would be highly advantageous to have, an active radiometric thermometry method that is independent of acquisition geometry and target emissivity.